The global positioning system (GPS) is a satellite-based radio-navigation system built and operated by the United States Department of Defense. The system uses twenty-four satellites orbiting the earth at an altitude of about 11,000 miles with a period of about twelve hours. More than twenty-four satellites may be present as spares. These satellites are placed in six different orbits such that at any time a minimum of six satellites are visible at any location on the surface of the earth except in the polar region. Each satellite transmits a time and position signal referenced to an atomic clock. A typical GPS receiver locks on to this signal and extracts the data contained in it. Using signals from sufficient number of satellites, a GPS receiver can calculate its position, velocity, altitude, and time.
The GPS receivers can acquire signals in many modes. In a “hot start” mode, the receiver already has the time, its last position, and the information on satellite position (also known in the art as almanacs or ephemeris) stored in its memory. The receiver can use this stored information to determine which satellites are probably visible, and it can then lock on to those satellite signals in a short time. On the other hand, the receiver may have no prior data on its position, time, or almanacs stored. In this “cold start” mode, the receiver has to search for signals from all of the satellites present in the constellation. There are some other modes where partial information on time, position and almanacs are available and corresponding start mode is known as “warm start.”
The GPS receiver has to acquire and lock on to at least four satellites in order to derive the position, velocity and time. Usually, a GPS receiver has many parallel channels, each receiving signals from a separate visible GPS satellite. The acquisition of the satellite signals involves a two-dimensional search of frequency and the PRN code phase. Each satellite transmits a unique PRN code, which repeats every millisecond. The receiver locally generates a replica frequency and a replica code phase and correlates these with the received satellite signals. The PRN code has to be searched in at least 2046 phases and the frequency search depends upon the Doppler frequency due to relative motion between the satellite and the receiver. Additional frequency variation may result due to local oscillator instability.
When the satellite signal is strong the receiver can detect the presence of a satellite signal in a short time. But when the signal is weak a long signal correlation is needed and the integration or correlation needs to be coherent which requires large computation load. The Signals may be weak due to an obstruction by foliage or buildings, or indoor operation. Special techniques are required to acquire the signal under these weak signal power conditions. One of the more widely used techniques under these conditions is known as assisted GPS (AGPS). In this method a cellular base station or server provides the ephemeris, time and data bit edge position to the GPS receiver in the cell phone so that it may acquire the satellite signal. This technique requires synchronization with the base station or server, and the service has to be provided by the cell phone operator. Consequently, it results in extra subscription charges and base station augmentation.
Due to the disadvantages with AGPS, it is desirable to be able to acquire weak GPS signals without outside assistance. Examples of this approach are disclosed in U.S. Pat. Nos. 5,271,034, 6,392,590, and 6611756. Most of these techniques, however, are not suitable when the signal is extremely weak due to the large computation involved in carrying out lengthy integrations and fast Fourier transforms (FFTs). In these techniques, the integration involves the summing of one-millisecond correlation values. A correlation value is obtained by comparing the sample values of input signal with locally available PRN code samples over a one-millisecond interval. The difference between the agreement and disagreement of the sample values is this correlation value. In the case of perfect correlation and no noise, the correlation value is equal to the number of samples in the one millisecond length, e.g., if the number of samples per code-length in one millisecond is 2046, then the perfect correlation value is 2046. But if the codes are not aligned this value may be −130 or +126 or −2. Thus, in this case the detection of the received signal can be determined easily. In the presence of noise, however, the correlation value may not be 2046, but may have a lower value, and when the signal is extremely weak it may not be able to determine the correct correlation. In these circumstances, the receiver can estimate the correlation value over several consecutive milliseconds to arrive at a reasonable value. This summing up over several milliseconds is also known as coherent integration. The coherent integration, however, requires that there are no sample reversals due to the residual carrier frequency. If there are reversals due to carrier frequency, the correlations may be carried out over non-reversed parts of the sample lengths and may be added by squaring each part. This is known as non-coherent integration. Compared to non-coherent integration, coherent integration provides better results for the same integration length. To boost the weak signal power, long time integration is necessary.
Once the satellite signal has been acquired, it is necessary to lock on to the signal by closely following the variations of the signal characteristics, which process is also known as tracking the signal. A receiver may fail to track an acquired signal due to reasons such as significant drop in the signal power, or a variation in the carrier frequency caused by satellite Doppler, local oscillator instability, or large platform dynamics.
In addition to tracking the signal by correlation with the PRN code, a GPS receiver also needs to demodulate data modulated on top of the PRN code signal at a slower bit rate. The GPS L1 signal is a code division multiple access (CDMA) signal which uses direct sequence to bi-phase modulate the carrier. The principal navigational signal L1 is spread by C/A code. One-millisecond correlation with corresponding pseudo-random noise (PRN) sequence is used for C/A code demodulation. In addition to the C/A code, the GPS signal also includes supplementary navigational data modulated at 50 bits/second using bi-phase shift key (BPSK) modulation. In BPSK, bits 1 and 0 are indicated by carrier phase shifts of 0 degrees and 180 degrees, respectively. This navigational data includes ephemeris and almanac data describing the satellite location, satellite health information, satellite clock bias, etc. Without this supplementary navigational information, correct user position and time cannot be determined. So it is necessary to demodulate this navigational data from received satellite signals.
After bit synchronization (i.e., after the bit edge has already been detected) the demodulation of the navigational data needs to determine bit polarity. Under weak signal reception conditions, the received signal power fluctuates with time. And in such cases, the bit polarity may not be correctly determined by known techniques, which use only information in the current data bit (i.e., 20 one-millisecond correlation values over the current data bit). There is therefore a need for improved methods for determining bit polarity.